Filament cutting devices were recently introduced and have rapidly become well known. A power source rotates a head or housing at high speed. One or more flexible, non-metallic filaments extend from the head or housing a short distance, the speed of rotation causing the filament to stiffen and thus constituting a cutting element.
Heavy duty trimmers for commercial use are typically equipped with a gasoline engine as a power source and may have a relatively heavy filament, often as a fixed length line which is replaceable as a unit. The fixed length filament may be on the order of 0.105 or 0.130 inch diameter line, whereas a trimmer designed essentially for use by a homeowner typically utilizes a filament of 0.080 inch diameter, or less.
The filament naturally wears and abrades as it encounters vegetation, fences, foundations, etc., and must be replaced periodically. As might be expected, the smaller filament wears more rapidly, and thus the trimmer utilizing this filament is often equipped with a reserve supply, which may be coiled in the head or housing, and is available as needed. The trimmer may have a manual system for unreeling more filament, or may have an escapement device to meter out filament while the device is in operation.
The larger diameter filament would normally be utilized on commercial trimmers, both because it lasts longer, and also because it can cut heavier vegetation which normally would not be encountered around a residence. Because of its relative stiffness, the larger diameter filament does not lend itself readily to being coiled on a small diameter spool as is necessary on a consumer-type trimmer, so a heavy filament of a fixed length is often used in the commercial units.
One such device is disclosed in U.S. Pat. No. 4,190,954, assigned to The Toro Company, assignee of the present invention. The trimmer head of this device is metallic, which requires balancing, thus adding to the cost. The filament is strung across the head in a serpentine manner and is retained by being wedged into slots in adjacent pairs of ear members on the bottom of the head. Ramps are required to prevent the filament from being abraded against the ground. The filament exits at the periphery of the head of this device, so the filament is often pinched between the head and an obstruction such as a wall or foundation or the like.
Another fixed length filament trimmer is disclosed in U.S. Pat. No. 4,054,992 to Ballas et al. FIG. 4 illustrates a grooved head where the grooves are slightly smaller than the filament, but are enlarged below the surface so the filament fits "in a relatively slideable manner" as revealed in Column 9, lines 31-35. The present invention retains the filament so it cannot slide or move axially.
It has been found that vibration develops in the filament from contact with vegetation and other obstructions, and unless controlled, results in an excess build-up of heat where the filament contacts the head. This heat can increase to the point where the filament literally melts and fuses itself to the head, or breaks off.
The present invention provides a balanced head of relatively small diameter which provides a practical means for retaining a fixed length filament of various diameters which is easily replaceable, and for minimizing vibration in the filament.